Both are cute and fuzzy and sometimes like to snuggle with you and be petted. BUT:
Un-neutered males of both species will spray urine to mark territory. Sometimes females will, too. Rabbit urine can be red, even when they are healthy so a splatter on the wall can look like a murder scene.
Cats can be left to roam the house generally unsupervised. They'll usually climb onto furniture/appliances whether you want them to or are aware they do. Sometimes they knock things over and break them if they're bored.
Rabbits will chew on inappropriate things and must have the area 'rabbit proofed' before being allowed to roam free in the house. Or you will have a dead rabbit (at the least) or a burned down house (at the worst) because they decided to chew an electrical cord. They also have habits of chewing up carpet/furniture and even walls out of boredom. That doesn't mean this -will- happen, it -might-. Because rabbits -need- to gnaw on things. So you have to give them things that they can chew on.
Cats and rabbits can be trained to use a litter box. Cats do it naturally in most cases but rabbits have to learn what is for pooping and peeing in. (Usually accomplished by putting a hay-rack over the bunny toilet. They tend to go poop/pee when they eat. Rabbit urine STINKS. (As does cat urine, but rabbits seem more likely to have accidents around the area than a cat does.)
Veterinarians who treat cats are common, but not all vets treat rabbits. Most can treat rabbits, but be sure to call around and make sure. You don't want a sick bunny and not know just where to rush them to for help! (I have a guinea pig and the only vet that treats them in my area is two towns and an hour long drive away!)
Both are excellent choices as pets, but they each come with their own troubles and special needs. :3
Cats gotta have a scratching post, rabbits gotta have hay (note that it's not 'like to', it's 'have to'.)
Which is another thing to consider...if you're allergic to hay, I'd lean hard for the kitty. I'm allergic to hay and have had non-stop breathing issues since I got my guinea pig. XD (I knew it going in, though, and I still think he's an adorable little fuzz nugget, but after having him for this long I don't think I would get another one.)
ALSO! You need a cage for the rabbit to sleep in/be in when you can't be nearby to make sure they aren't getting into trouble. And bigger is better. My guinea pig has an eight square foot 'house'. (4ft long x 2ft wide)
I've gotta go to work now, so I have to cut this shorter than I would like. If you've got any other questions, just ask! I've had so many different animals in my life that I can tell you what the hardest part of having them is going to be. (If you can get over the bad, you'll get to the 'fun'. :3)